1. Field of the Invention
This invention relates to a multiple channel fiber optic communications system, and in particular to a multitier wavelength division multiplexing arrangement therefor.
2. Discussion of Related Art
Fiber optic systems are presently being used for high bandwidth, high speed voice and video communications. Originally, single channel systems in which each fiber carried a single channel sufficed, but increases in traffic have led to a need for greater channel-carrying capacity. Because of the high cost of laying optical fibers, increasing capacity by laying more fibers is impractical, and thus achieving greater efficiency in utilizing fiber resources has become increasingly important, which has led to the development of systems for adding channels to existing fibers.
The most practical method of carrying multiple channels over a single fiber is to create channels having different non-interfering wavelengths. The different wavelength carriers are coupled to the single fiber using a technique known as wavelength division multiplexing and separated at the receiving side by demultiplexing the signals. While such techniques have greatly increased the capacity of existing fiber optic systems, the traffic demands on such systems have risen even faster, resulting in the need to further increase the number of channels which can be carried.
Since the total bandwidth of the signals which can be carried by an optical fiber is limited, adding channels to existing fibers involves increasing the channel density and decreasing the channel separation. The smaller the channel separation, however, the more difficult it is to isolate the channels, and to separate them without significant losses. Nevertheless, current technologies are capable of providing for two-way communications over channels separated by less than one nanometer, using a technique known as dense wavelength division multiplexing (DWDW).
Copending U.S. patent application Ser. No. 08/921,330, filed Aug. 29, 1997, pending, describes a variety of components suitable for use in DWDM systems, based on the principle of using Mach-Zehnder interferometric filters having in-fiber Bragg gratings to insert and remove individual channels from a multiplexed signal. The multiplexed interferometric filters are arranged in serial or cascaded fashion and packaged in modular components having four or eight filters each.
While the number of filters in a component is theoretically unlimited, however, and any number of components can be cascaded together, in practice the number of filters that can be connected together in this fashion is limited by the additive effect of the losses due to the individual Bragg gratings through which a signal must pass after insertion and prior to separation.
An alternative fiber optic wave division multiplexing technique is to use multiple port optical circulators having in-fiber Bragg gratings at the single channel input and output ports to insert and remove individual channels from the multiplexed signal. Optical circulators are commercially available items and their use at least in demultiplexers is described in U.S. Pat. No. 5,608,825. However, for many applications, Mach-Zehnder couplers of the type described in the above-cited copending application are preferred to optical circulators.
The present invention addresses the problem of limitations as to the number of individual filter units that can conveniently be cascaded without unacceptable losses by proposing to use optical circulators as combiners, also known as collectors, for the cascaded individual channel multiplexing and demultiplexing components described in the copending patent application. Because of the their modularity, the components can easily be added to a suitably arranged combiner, thereby obtaining the advantages of superior channel isolation while adding a degree of parallelism in order to limit losses as the number of channels is increased.
This use of combiners is similar to the proposal made in the copending application to existing narrowband wavelength division multiplexing (NWDM) components as combiners for the outputs of the individual cascaded multiplexer components, and may be considered an extension of the concept, but the use of optical circulators as combiners has the advantage of providing a greater passband for the combined channels, allowing the entire bandwidth of the individual cascaded multiplexer components to be utilized.
Thus, although optical circulators and Bragg gratings are well-known technologies, as is their combination in WDM devices, their use as combiners in combination with cascaded or serial individual channel separating arrangements is both novel and provides advantages unattainable using current optical circulator arrangements.